1. Field of the Invention
The present invention relates to a plate-like objects detector for detecting the state of each of plate-like objects (or detecting their presence, number, positional shift from their positions and the like) in the case where the plate-like objects such as semiconductor wafers, LCD substrates and printed boards are aligned in a column and processed as a batch.
2. Description of the Related Art
A plurality of semiconductor wafers are housed in a cassette and the wafers are carried as a lot from a place where a process is conducted to another place where a next process is to be conducted in the course of manufacturing semiconductor devices. 50 sheets of the wafers are batch-processed while immersing them in liquid as a lot in the washing and cleaning system, for example. In order to normally conduct the process, it is needed that the number of the wafers can be kept as previously determined and that they can be aligned as previously expected.
Jpn. Pat. Appln. KOKAI publication No. 62-133286 discloses a wafer counter having right emitting and receiving sections and a transparent infrared laser oscillator. In the wafer counter, a cassette is moved up and down and the number or the position of infrared laser beams interrupted by wafers is counted, thereby detecting the number or position of wafers in the cassette.
In this type of wafer counter, an infrared laser beam is reflected by the peripheral edge of a wafer, and the reflected light beam is applied to another wafer (an upper or lower wafer). Further, the reflected light beam becomes a number of scattered light beams, which are detected by the light receiving section, resulting in that the detection accuracy is greatly decreased.
If the number of wafers in the cassette is not accurately counted, the following inconveniences may occur when wafers are transferred in or from the cassette: it is impossible to hold wafers in the cassette without a vacant pocket (holding groove); and the detector erroneously detects that the cassette is vacant in a case where an unprocessed wafer remains in the cassette.
If the position of wafers is not accurately detected, a handling arm of a robot cannot hold wafers at a right position and the wafers cannot be inserted in the pockets (holding grooves) in the cassette. In this case, wafers may be damaged or the cassette may fall down.
Jpn. Pat. Appln. KOKAI publication No. 60-213209 discloses a positioning apparatus for directing the orientation flat of a wafer in a desired direction by use of an image sensor. In the positioning apparatus, the light emitting and receiving sections of an image sensor are fixed so as to face each other. When a wafer interrupts a infrared laser beam belt emitted by the light emitting section, the width of the interrupted light or a transparent light is detected by the light receiving section. Wafers are positioned based on the detected data so that the orientation flat is directed to a desired direction.
However, in the above positioning apparatus, since an infrared laser beam is reflected by the peripheral edge of a wafer and the reflected light beam is applied to inner walls of the apparatus and scattered, scattered light beams are detected by the light receiving section. Moreover, the wafer cannot be aligned at a predetermined position or wafers are shifted due to influenced of a noise signal of the scattered light beam.
Further, if shifted wafers are transferred into a plasma etching apparatus, the wafers cannot be placed at a right position of a static chuck (table). Hence, the static chuck is damaged by plasma. In addition, the directions of orientation flats of the wafers are different, resulting in variance of processes on the wafers.